Point oxygen vacancy

An effect which is frequently encountered in oxide catalysts is that of promoters on the activity. An example of this is the small addition of lidrium oxide, Li20 which promotes, or increases, the catalytic activity of dre alkaline earth oxide BaO. Although little is known about the exact role of lithium on the surface structure of BaO, it would seem plausible that this effect is due to the introduction of more oxygen vacancies on the surface. This effect is well known in the chemistry of solid oxides. For example, the addition of lithium oxide to nickel oxide, in which a solid solution is formed, causes an increase in the concentration of dre major point defect which is the Ni + ion. Since the valency of dre cation in dre alkaline earth oxides can only take the value two the incorporation of lithium oxide in solid solution can only lead to oxygen vacaircy formation. Schematic equations for the two processes are... 

The oxygen vacancies then diffuse to the gas interface where they are annihilated by reaction with adsorbed oxygen. The important point, however, is that metal is consumed and oxide formed in the same reaction zone. The oxide drift has thus only to accommodate the net volume difference between the metal and its equivalent amount of oxide. In theory this net volume change could represent an increase or a decrease in the volume of the system, but in practice all metal oxides in which anionic diffusion predominates have a lower metal density than that of the original metal. There is thus a net expansion and the oxide drift is away from the metal. 

S. 3 Water Dissociation at Oxygen Vacancies and the Identification of Point Defects 221 8.3... 

A number of the well-known y-induced centers in n-type Si are also neutralized by atomic hydrogen (Pearton, 1982). The A-center (oxygen-vacancy complex, Ec-0.18eV and divacancy level (Ec-0.23eV) are passivated, while the E-center (phosphorus-vacancy complex, Ec -0.44 eV) is thermally removed at relatively low temperatures and its susceptibility to hydrogenation could not be determined. Point defects... 

The conductivity of titanium dioxide, TiC>2, at 1166 K was found to depend upon the oxygen partial pressure as in the following table. Assuming that oxygen vacancies are introduced into the oxide as the main point defect, is the vacancy more likely to be doubly or singly charged ... 

Thermodynamic considerations imply that all crystals must contain a certain number of defects at nonzero temperatures (0 K). Defects are important because they are much more abundant at surfaces than in bulk, and in oxides they are usually responsible for many of the catalytic and chemical properties.15 Bulk defects may be classified either as point defects or as extended defects such as line defects and planar defects. Examples of point defects in crystals are Frenkel (vacancy plus interstitial of the same type) and Schottky (balancing pairs of vacancies) types of defects. On oxide surfaces, the point defects can be cation or anion vacancies or adatoms. Measurements of the electronic structure of a variety of oxide surfaces have shown that the predominant type of defect formed when samples are heated are oxygen vacancies.16 Hence, most of the surface models of... 

In fluorite-structure oxygen-deficient oxides MO2-X there is general agreement that the oxygen vacancy is the point-defect responsible for non-stoichiometry. Unfortunately, no direct observation is available of a basic cluster species for MO2-1 such as the Willis cluster in UO2+X. 

The important point for bonding is not so much the structural description of the strongly disturbed oxygen sublattice, but (as discussed for U02+x), the required association postulated between ions and oxygen vacancies. Once again, molecularities are formed, strongly hinting at covalent effects. 

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